The present invention relates to a laser apparatus and, particularly, to a laser apparatus capable of producing a high power and high quality laser beam.
FIG. 49 is a cross section of an example of a conventional laser apparatus such as shown in Hamazaki "Practical Laser Machinings", 1986, Tech Publishing Co. In FIG. 48, a reference numeral 1 depicts a convex laser output mirror matrix having a full reflection coating 40 on a center portion of an inner surface thereof, an annular non-reflection coating 2 on an outside of the full reflection coating 40 and a non-reflection coating 2 on an outer surface thereof. 6 depicts a concave collimating mirror and 7 a laser medium such as CO.sub.2 gas to be excited by, for example, electric discharge or a glass material to be excited by such as flash lamp when the laser apparatus is of solid state type. Reference numerals 8 and 9 depict laser beams produced in an unstable resonator composed of the mirrors, an output laser beam being shown by a reference numeral 10.
The laser beam 8 fully reflected and enlarged by the full reflection coating 40 on the inner surface of the output mirror is collimated by the collimating mirror 6 to parallel laser beam 9 while amplified by the laser medium 7 and an annular ring shaped laser beam 10 is derived from an outside portion of the full reflection coating 40 as an output beam having a distribution pattern such as shown in FIG. 50a and a substantially uniform phase and a portion of the laser beam fallen in the coating 40 is reflected back to the resonator in which it reciprocates while being amplified.
In order to utilize such ring shaped output laser beam for a laser machining, it is necessary to condense it by a lens system. In such case, however, it is very difficult to condense it without sidepeaks as shown in FIG. 50b due to diffraction effect of laser ring. The magnitude of the sidepeak is determined by the magnification factor M which is the ratio of an outer diameter to an inner diameter of the ring, the larger M value providing the better condensation. Therefore, the sidepeaks can be minimized by reducing an outer diameter of the full reflection coating 40. In such case, however, a considerable portion of laser energy is derived every reciprocation of beam in the resonator, resulting in a degradation of coupling rate of the resonator and a degradation of oscillation efficiency. Therefore, the upper limit of the industrially usable M value is usually about 2 at most.
Further, since the partial reflection mirror matrix 1 is heated non-uniformly by the ring shaped output beam, the matrix 1 is subjected to a non-uniform internal thermal stress, causing a phase distribution of the output beam to be disordered.
Further, when it is desired to obtain a higher output laser beam, there is a strong possibility of damage of the full reflection coating 40 since beam intensity in a center portion of the resonator becomes high.
In case of the solid state laser device, there are various phase distributions in the laser beam reciprocating in the resonator. Among others, the so-called TEM.sub.oo mode beam is considered as suitable for use in laser machining. However, TEM.sub.oo mode laser beam has a small angle of divergence and the cross section thereof is the minimum among others. Therefore, in order to select this mode, it is necessary to arrange an aperture member having a small opening in an optical path. For the case of YAG laser having radii of curvature of the full reflection mirror 6 and the output mirror 1 being 20 m, respectively, and an optical distance therebetween being 1 m, as an example, a diameter of TEM.sub.oo laser beam within the resonator is 1.8 mm since wavelength of the laser is 1.06 .mu.m. That is, the laser machining must be performed with very small power obtainable from such small diameter laser beam. In order to derive a power high enough to perform a desired machining, it is necessary to increase beam intensity within the solid medium, causing the latter to be distorted. Thus, the beam quality is degraded and, in some cases, a multimode is generated. Therefore, the maximum output power of the conventional solid state laser device has been 10 W to 20 W, at most.